Add missing override keyword:

* Enable the `suggest-override` warning for gcc
* Fix all functions that were flagged by that warning

src/test/protocol/BuildInfo_test.cpp

@@ -84,7 +84,7 @@ public:
         BEAST_EXPECT(to_packed (from_version (65535, 65535)) == 4294967295);
     }
 
-    void run ()
+    void run () override
     {
         testValues ();
         testStringVersion ();

src/test/protocol/IOUAmount_test.cpp

@@ -246,7 +246,7 @@ public:
 
     //--------------------------------------------------------------------------
 
-    void run ()
+    void run () override
     {
         testZero ();
         testSigNum ();

src/test/protocol/InnerObjectFormats_test.cpp

@@ -171,7 +171,7 @@ static TestJSONTxt const testArray[] =
 class InnerObjectFormatsParsedJSON_test : public beast::unit_test::suite
 {
 public:
-    void run()
+    void run() override
     {
         using namespace InnerObjectFormatsUnitTestDetail;
 

src/test/protocol/Issue_test.cpp

@@ -456,7 +456,7 @@ public:
 
     //--------------------------------------------------------------------------
 
-    void run()
+    void run() override
     {
         testcase ("Currency");
         testUnsigned <Currency> ();

src/test/protocol/Quality_test.cpp

@@ -347,7 +347,7 @@ public:
         BEAST_EXPECT(qa < qb);
     }
     void
-    run()
+    run() override
     {
         test_comparisons ();
         test_composition ();

src/test/protocol/STAmount_test.cpp

@@ -606,7 +606,7 @@ public:
 
     //--------------------------------------------------------------------------
 
-    void run ()
+    void run () override
     {
         testSetValue ();
         testNativeCurrency ();

src/test/protocol/STObject_test.cpp

@@ -632,7 +632,7 @@ public:
     }
 
     void
-    run()
+    run() override
     {
         // Instantiate a jtx::Env so debugLog writes are exercised.
         test::jtx::Env env (*this);

src/test/protocol/STTx_test.cpp

@@ -31,7 +31,7 @@ namespace ripple {
 class STTx_test : public beast::unit_test::suite
 {
 public:
-    void run()
+    void run() override
     {
         testcase ("overly nested transactions");
         testDeepNesting();
@@ -1292,7 +1292,7 @@ public:
 class InnerObjectFormatsSerializer_test : public beast::unit_test::suite
 {
 public:
-    void run()
+    void run() override
     {
         auto const kp1 = randomKeyPair (KeyType::secp256k1);
         auto const id1 = calcAccountID(kp1.first);

src/test/protocol/STValidation_test.cpp

@@ -108,7 +108,7 @@ public:
     }
 
     void
-    run()
+    run() override
     {
         testDeserialization();
     }

src/test/protocol/XRPAmount_test.cpp

@@ -190,7 +190,7 @@ public:
 
     //--------------------------------------------------------------------------
 
-    void run ()
+    void run () override
     {
         testSigNum ();
         testBeastZero ();

src/test/protocol/digest_test.cpp

@@ -148,7 +148,7 @@ public:
         pass ();
     }
 
-    void run ()
+    void run () override
     {
         testSHA512 ();
         testSHA256 ();